So as previously mentioned parts for this thing are basically unobtainable here. However all the parts are replaceable with modern alternatives with the exception of the check valve. This is a simple flat disc cup 0.405 OD x .110" thick. The original is made of brass with a (delrin?) insert. It can be replaced by a simple piece of delrin of the same dimensions. I choose to remake the piece as it was originally because this is a customers rifle.
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The valve poppet can be replaced by a disco poppet. I had one but unfortunately the old seat was so cockeyed that it ruined the my only disco poppet so I had to make one. I do this by drilling an undersize hole in the delrin then spinning the stem at warp speed while pushing the delrin head on. This fuses the two pieces together then you machine the face and dress down the head to the right shape
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The lead seals can be replaced by (2) 015 orings. The first oring fits onto the valve as seen in the last pic. The second oring gets squashed between the cap nut and the valve core. Lots of threads out there say this seal is unnecessary but if it's not there the transfer port will leak.
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From this point the only difficulty is getting the little tab on the valve core lined up with the slot in the body that's soldered into the tube while simultaneously compressing the valve core spring and screwing in the cap nut...tricky but it works with the tool.